β-adrenergic regulation of glucose transporters

The transport of glucose across the plasma membrane is a fundamental mechanism to provide cells with its basic requirements for energy yielding processes. It is also vital for clearing glucose from blood into tissues, a process normally stimulated by the hormone insulin in mammals. The sympathetic nervous system, normally activated during stress, also regulates glucose transport. The sympathetic neurotransmitter noradrenaline, acts on the family of adrenergic receptors (ARs). An important subtype of the AR family is the β-AR, which is subdivided into the β1, β2, and β3-AR. Glucose is transported across the plasma membrane by the family of glucose transporters (GLUT1-12, and HMIT). In this thesis, I have investigated the β-AR regulation of GLUT1 and 4, and glucose uptake, in skeletal muscle cells and brown adipocytes in culture, model systems which correspond to metabolically active, sympathetically innervated and insulin-sensitive tissues. In brown adipocytes, activation of the β3-ARs induced the expression of GLUT1, resulting in a large increase of glucose uptake. In skeletal myotubes, we postulate there is a possible mechanism where β2-ARs can regulate the intrinsic activity of GLUT1. We found that insulin signaling, but not β-adrenergic signaling, mediated glucose uptake through class I phosphatidylinositol 3-kinase (PI3K). The β-adrenergic signaling to glucose uptake appeared to involve a PI3K related kinase (PIKK), in both skeletal myotubes and brown adipocytes. Furthermore, the increase of glucose uptake by β-ARs in brown adipocytes is partially mediated by AMP-activated protein kinase (AMPK). However, in an artificially constructed system, with cells expressing GLUT4 and β2-ARs, both insulin and β-adrenergic activation translocated GLUT4 and increased glucose uptake. These results show that β-adrenergic signaling increase glucose uptake by regulating glucose transporters through distinct pathways, in skeletal myotubes and brown adipocytes